Magnetic field gradiometer



Aug. `15, 1961 .1. G. FERGUSON MAGNETIC FIELD GRADIOMETER Original FiledAug. 17, 1944 6 Sheets-Sheet 1 ATTORNEY Aug. 15, 1961 J. G. FERGUSONMAGNETIC FIELD GRADIOMETER 6 Sheets-Sheet 2 Original Filed Aug. 17, 1944NVENTOR By J. G. FERGUSON @amm 7m ATTORNEY Aug. 15, 1961 J. G. FERGUSONMAGNETIC FIELD GRADIOMETER v #l Ok Original Filed Aug. 17, 1944 www? 7mATTORNEY Aug. 15, 1961 J. G. FERGUSON 2,996,563

MAGNETIC FIELD GRADIOMETER Original Filed Aug. 17, 1944 6 Sheets-Sheet 4Wvg/v70@ J G. FERGUSON BV www W 7W ATTORNEY TO FG. 3

Aug. l5, 1961 J. G. FERGUsoN MAGNETIC FIELD GRADIOMETER 6 Sheets-Sheet 5Original Filed Aug. 17, 1944 mw NG m WF a J.

ATTORNEY Aug' 15 1961 J. G. FERGUSON 2,996,663

MAGNETIC FIELD GRADIOMETER Original Filed Aug. 17, 1944 6 Sheets-Sheet 6/Nl/E/vro/P JG. FERGUSON www, 7n 7m AT TORNEV United States liatent i 3Claims. (Cl. 324-43) This invention relates to magnetic detectionsystems and more particularly to a magnetic detector for a magneticfield gradiometer system responsive to the difference in magnetic fieldintensity between two points in space. This application is a division ofmy original application, Serial No. 549,872, Ifiled August 17, 1944.

Magnetic field measurements, especially in the study of terrestrialmagnetism, may comprise measurements of the horizontal field intensity,the vertical field intensity, the angles of inclination and declination,measurements of the total field intensity and measurements of fieldgradient. In the measurement of the field gradient, it is sometimesdesirable to know only the absolute value of the gradient along aparticular axis. In other applications it is necessary that not only:the absolute value of the field gradient be known but also itsalgebraic sign, that is the direction in which the field is increasingor decreasing along the specified axis. Either the direction ofincreasing or the direction of decreasing field may be regarded ashaving the positive algebraic sign, the other direction being regardedas negative.

It is the object of this invention to provide a magnetic detector for amagnetic rfield gradiometer system of great sensitivtiy, capable ofindicating either the absolute value of a small field gradient or bothits magnitudeand its direction or algebraic sign, the system also beingadapted for indicating the average intensity of the field componenttaken along the axis where the field gradient measurement is made.

The foregoing object is attained by this invention by providing agradiometer detector comprising three magnetometer elements, eachcomprising a core of magnetic material with windings thereon and eachhaving a principal magnetic axis, and a supporting means for supportingthe three magnetometers in a fixed spaced relationship with theirprincipal axes aligned to substantial parallelism.

The invention may be better understood by referring to the accompanyingdrawings in which:

FIG. l discloses a block schematic of one embodiment of this inventionadapted for indicating the absolute magnitude of the field gradient in aparticular direction;

FIG. 2 is another block schematic diagram disclosing a differentembodiment of the invention adapted for indicating not only themagnitude of the field gradient but also its algebraic sign;

FIGS. 3 and 4 are more detailed circuit diagrams of the circuitsemployed in the gradiometer system of FIG. 2; and

FIGS. 5 to 8 inclusive, show various views of the mechanical assembly ofthe gradiometer field detector unit.

Referring now more particularly to FIG. 1, there is shown a fielddetector unit denoted generally by the reference numeral 1. This`detector unit comprises two gradiometer detector units f2 and 3 and anaverage field detector unit 4. Each of these units comprises a core 5 ofmagnetic material having wound thereon a detector coil 6 and a feedbackcoil 7. The detector coil is preferably wound around the core and thiswound unit is inserted inside the feedback coil. This will be moreparticularly described later in connection with FIG. 6A. The twogradiometer units or magnetometers 2 and 3 are Patented Aug. 15, 1961preferably spaced `apart a known fixed distance between their magneticcenters which distance is denoted AX on FIG. 1. It will be obvious thatshould the gradiometer units 2 and 3 be equally sensitive to a field ofthe same intensity, the difference in response between these twoelements when placed in a uniform `field would be zero. It is equallyobvious that if the magnetic field is different they will have unequalresponses so that their difference in response will not be zero. Thedifference in field intensity at the two gradiometers may be denoted bythe symbol AH. The magnetic field gradient is then expressed by theratio AH/AX and as the response of the two gradiometers will differ inproportion to the field difference AH the difference in response is ameasure of the gradient. .The manner in which these gradiometer unitsrespond to the magnetic field component will be more particularlydescribed later.

Collinearly with the gradiometer-detector units 2 and 3 and preferablyintermediate therebetween is the average field detector unit 4. Thisunit is preferably located so that its magnetic center is equidistantfrom the magnetic centers of the gradiometer detector units 2 and 3.Since the field component at this point is substantially equal to theaverage of the field components existing at the gradiometer-detectorunits 2 and 3, -it is obvious that the response of this element would beproportional to the average field component acting in the direction oftheir common principal axis.

The three detector units 2, 3 and 4 are substantially identical in theirconstruction. As previously stated each of them contains a magnetic core5 upon which the windings 6 and 7 are wound. One of the windings on eachof the detector units is used for energizing the magnetometer unit witha current of fundamental frequency. In gradiometer-detector unit 2,winding '6 is used for this purpose and if a current of fundamentalfrequency is passed through this winding of sufficient magnitude todrive the magnetization in core 5 near saturation even order harmonicvoltages will appear across windings 6 and 7 proportional in magnitudeto any constant component of the magnetic field which may be acting inthe direction of the principal axis of the magnetic core. Actually botheven and odd order harmonics are generated in all windings surroundingthe core including the Winding used for energizing the core. Theproduction of harmonics is due to the curvature of the magnetizationcurve and therefore the core is excited close to saturation. For anygiven value of external field, the output of any single even orderharmonic increases rapidly as the exciting voltage is increased untilthe knee of the curve is reached and reaches a rather broad maximum asthe exciting voltage is further increased, the value of maximum outputbeing a function of the order of the harmonic. The harmonic outputvoltage is approximately proportional to the signal field provided thatthe signal field is within the linear portion of the magnetization curveof the core. These relationships may be expresed by the followingmathematical equation:

VoiKHJfE) (1) V0 is the generated output voltage of any selected evenorder harmonic.

K is a constant depending upon the core material, itsy As previouslystated these even order harmonic voltages are generated in each andevery winding surrounding the magnetometer core, as for example,windings 6 and 7 which surround core 5. It is obvious that additional awindings may also be employed in which any one or more windings may beutilized tfor exciting the magnetom. eter while the others may be vusedfor detecting different ones of the even order harmonics. In FIG. 1,winding 7 is actually employed as a compensating winding to reduce theamount of eld affecting the core 5, the amount of reduction being inproportion to the strength of the average field component. The manner inwhich this is done will be `described in greater detail later. Insteadof using a separate winding for picking up the generated even orderharmonics, winding `6 is used for both energizing the magnetometer 2 andalso as the pick-up winding for detecting the generated even orderharmonics.

The voltage used for exciting the magnetometer may be of any convenientfrequency as for example l kilocycle. In FIG. l the source of thisfrequency is the 1 kilocycle oscillator 8 which passes current to theexciting windings of the magnetometers through a 1 kilocycle band-passfilter 9. The output of this filter is taken from terminals 10, 11 and12, the common return to the filter being the ground terminal 13.Band-pass filter 9 may be of any of the well-known band-pass filtertypes and is designed to have three output channels feeding the threeterminals 10, 11 and 1:2, respectively. Current `from terminal passesover lines 14 and 20 to the detector energizing coil `6 of matgnetometer2 and thence back to the ground terminal 13 through an obvious circuitincluding ground. The energizing coil of gradiometer unit 3 is energizedover a similar circuit from filter terminal 11 by way of conductors 15and 21 and back through ground to terminal 13 of the filter. Likewise asimilar circuit to the average field detector 4 starts `from terminal 12of the filter over conductors lt? and 212 through the exciting windingand back to terminal 13 of the filter by way of ground. Reference arrowson conductors 20, 21 and 22 labeled 1 kc. indicate the passage of theseenergizing currents.

These energizing coils, as for example coil 6 for magnetometer 2 arealso used as pick-up coils for the even order harmonic voltagesgenerated in these magnetometers. For illustrative purposes the secondharmonic lof two kilolcycles may be selected, the flow of this currentbeing represented by an arrow labeled 2 kc. on conductors 20, 21 and 22,respectively. Gradiometer-magnetometers 2 and 3 are connected so thatthe second harmonics generated therein are in opposite phase withrespect to the series circuit formed by their pick-up coils and theprimary winding of transformer 23. Consequently, no second harmoniccurrent will actually flow through this series circuit unless one of thevoltages becomes larger than the other. With no current flowing,obviously there will be no voltage generated in the windings oftransformer 23 and consequently there will be no current in thesecondary of this transformer. However should the field components beunequal at the two gradiometer-magnetometers 2 and `3, the secondharmonic voltages generated in their windings will be unequal which willresult in a second harmonic current of two kilocyoles flowing throughthe primary winding of transformer 23. The primary winding oftransformer Z3 is connected to conductor 20= by way of conductor 17 andconductor 21 by way of conductor 18 so that the series circuit justreferred to is easily traced from the lower end of the primary oftransformer 23 through conductor 18, conductor 21 through the excitingpick-up winding of magnetometer 3, through the exciting pick-up coil ofmagnetometcr 2 through conductors 20 and 17 and back to the primarywinding of transformer 23. When the absolute magnitude of the fieldgradient at the field detector unit becomes greater than zero, thissecond harmonic current will flow through the primary of transformer 23thereby generating a voltage across its secondary. This voltage producesa current which passes through a 2 kilocycle band-pass filter 24, isamplified by a tuned amplifier 25, rectified by a rectifierv 26 andappliedv to a gradient indicator 27. The

gradient indicator may be any suitable direct current indicatinginstrument or in the alternative may be a recorder responsive to thedirect current from the rectifier 26.

The description thus far of the detector circuit operation neglects theeffect of the average field detector magnetonieter 4. Without this unit,precision measurements of small field gradients would be very difficultif not impossible from a practical point of view. This magnetometer 4 ispreferably located midway between the magnetic centers ofgradiometer-magnetometers 2 and 3 and is excited from terminal 12 ofband-pass filter 9' by way of conductors 16 and 22 through the excitingcoil and back to the grounded terminal 13 of the band-pass filter 9. Dueto the location of this magnetometer the second harmonic voltagesgenerated therein will be proportional to the average field componentexisting between the two gradiometer units 2 and 3. The second harmonicvoltage of two kilocycles generated therein in response to this averagefield will be carried over conductor 22 and through conductor 19 to thetwo kilocycle band-pass filter 28. The output from filter 28 isamplified by the tuned amplifier 29, is rectified by rectifier 30 andthe direct current output of this rectifier is passed through an averageeld indicator 31 and through the three feedback coils 7, 7, 7 of thefield detector unit. The return path `from the feedback coils is by wayof ground through an obvious circuit. The average field indicator 31 maybe a direct current instrument similar to the gradient indicator 27.Since as already indicated the second harmonic voltage output from theaverage field detector 4 is proportional to the average field intensity,the current output from rectifier 30 also varies in proportion to thestrength of this average field and consequently the average fieldindicator 31 may be calibrated in proportion to the average fieldintensity.

' As this current also passes through the three feedback coils 7, 7, 7in the field detector unit it provides a neutralizing or compensatingeffect.

This neutralizing effect is secured by having the feedback coils producea magnetomotive force acting in direct opposition to the field beingmeasured. In order for this to work properly, it is essential that thefeedback coils 7, 7 of the gradiometer units 2 and 3 be exactly alike toa very high degree of precision. It should be remembered that thisneutralizing current is proportional to the average field componentsince the direct current used for energizing these coils is derivedlfrom the second harmonic generated in the average field magnetometer 4.Consequently, if the average field changes, the neutralizing effect fromthe feedback coils 7 will correspondingly change. The advantagesrealized in using these feedback coils is better understood byconsidering that Without the feedback coils it would be necessary tomake the pick-up windings in the gradiometer units 2 and 3 exactly aliketo within at least one part in one hundred thousand in order to have thedetector operate with an error of less than one gamma in a uniform fieldof l gauss in the direction of the common axis of the field detectorunit. This requirement :is much more easily met in a practical system byutilizing the feedback coils to compensate or neutralize the greaterpart of the field acting on the gradiometer unit and since this feedbackmagnetomotive force is exactly equal for each of these twogradiometer-detectors 2 and 3, the difference between their outputsremains unchanged by reason of having used the feedback coils 7. This ineffect transfers the severe balance requirements from the gradiometerunit to the feedback or neutralizing coils where they can be met muchmore easily.

ln order to derive the maximum benefit from the feedback effect it isdesirable that the feedback coils 7, 7 surrounding thegradiometer-magnetometers 2 and 3 should contain a sufficient number ofturns to effectively reduce the field in these elements to zero when thefield detector unit is placed in a perfectly uniform field. It isobvious that the number of turns inthe feedback coil' 7 surrrounding theaverage field magnetometer 4 cannot be equal to nor as large as thenumber of turns in the feedback coils surrounding thegradiometer-magnetometers 2 and 3 for if the number of turns were equalit would be impossible for the feedback circuit to reduce the field inthe two gradiometer-magnetometers 2 and 3 to Zero. In actual practice ithas been found desirable in one embodiment of the invention to use 1.2percent more turns per inch in the feedback coils 7, 7 around thegradiometer-magnetometers 2 and 3i than are used in the feedback coil 7around the average field magnetometer 4.

To fix more firmly in mind the operation of the gradiometer systemdisclosed in FIG. l, its operation may be reviewed very briefly. Currentof fundamental frequency from the 1 kilocycle oscillator 8 is passedthrough the band-pass lter 9i to energize the three magnetometers 2, 3and 4, magnetometers 2 and 3` comprising the gradiorneter-magnetometersand magnetometer 4 being responsive to the average field. In a field ofuniform intensity, the second harmonic voltage outputs from thegradiometer-magnetometers Z and 3 are equal and since they are in phaseopposition, the net Voltage appearing across conductors and 2,1 will bezero so the gradient indicator Z7 will indicate zero. Should, however, afield gradient exist bet-Ween the elements Z and 3, these generatedsecond harmonic voltages will be unequal resulting in a current ofsecond harmonic frequency passing through the primary of transformer 23.The voltage in the secondary of transformer 23 responsive to this secondharmonic current in the primary will be filtered by filter 2.4,amplified by amplifier 251, rectified by rectifier 26 and indicated bythe gradient indicator Z7. The indicator may be calibrated in suitableunits as, for example, gamma per foot.

By reason of its location the second harmonic voltage output ofmagnetometer 4 is proportional to the average field existing between thegradiometer-magnetometers 2 and 3. This second harmonic output isfiltered by filter 28, amplified by amplifier 29, rectified by rectifier30 and the resulting direct current which is proportional to the averagefield is indicated by the average field indicator 31. This same currentpasses through the three feedback coils 7, 7, 7 surrounding the threemagnetomcter elements. In a field of uniform intensity the feedback issuch as to reduce the component of field in thegradiometer-rnagnetotmeters 2i and 3 to zero. In order to produce thiscurrent, however, the magnetomotive force generated in the feedback coil7 surrounding the average field detector 4 is less than around the twogradiometer coils thereby leaving a small residuum of field in theaverage field detector. In a field of unequal intensity where a gradientexists between the gradiometer units Z and 3 the compensating feedbacksupplied by the feedback coils 7, 7 surrounding the gradiometer units 2and 3 is still maintained rigorously equal for these two elements.Consequently the neutralizing or feedback coils do not affect in any waythe difference between the second harmonic outputs of these twomagnetometers created by the difference in the fields between them.

As previously stated in connection With FIG. 1 the gradiometer-indicator2.7 is responsive only to the absolute magnitude of the axial componentof field gradient and gives no indication what ever of the algebraicsign of this gradient. In order to provide an indication of thealgebraic sign of the field gradient it is necessary to use a detectioncircuit which is also sensitive to the phase of the unbalancedopposition voltage created in the coils 6, 6 of the gradiometer units 2and 3. Circuits capable of performing this function are shown in blockschematic form in FIG. 2. In this figure corresponding parts of the sameapparatus bear the same reference numerals as in FIG. 1. The fielddetector unit 1 is substantially identical to the one shown in FIG. 1with the single exception, however, that an additional adjusting coil 7-is shown connected in series with the feedback coils 7, 7, 7. Thepurpose of this adjusting coil is to aid in bringing to exact equalitythe feedback magnetomotive forces acting in the two end magnetometerelements Z and 3. This adjusting coil comprises a few turns of wirewhich may not only be varied as to its number of turns but also it maybe turned end for end so as to reverse its magnetomotive force. Thisadjusting coil s then mounted adjacent the end of one of the endfeedback coils as, for example, the upper one shown in FIG. 2 andadjusted until the two feedback coils have equal feedback magnetomotiveforces.

The exciting pick-up coils 6, 6, 6 of the three magnetometers Z, 3 and 4are energized from a source of fundamental frequency 8 which is filteredby a bandpass filter 9 and transmitted to these windings by way ofconductors 14, 15, 16, 2.0', 211 and 22. This is substantially identicalwith the arangement shown in FIG. l. However, `in order to cause thecircuit to respond to a phase reversal in the resultant second harmonicvoltages generated by the magnetometers 2 and 3 it is necessary tocompare this phase with some reference voltage of the same frequency.Also referring again for the moment to FIG. 1 it will be noted that thedirect current providing the feedback will not reverse in sign eventhough the field detector unit 1 is reversed with respect to the fieldbeing measured. While this makes no difference where the apparatus is tobe used in a fixed location it is intolerable where the apparatus is tobe iiown in an airplane and the direction of the principal axis of thefield detection unit 1 would be constantly changing. Consequently, notonly must the apparatus indicate the phase reversals of the detectedsecond order harmonic voltages from the gradiometer-magnetometers 2 and3 but the feedback magnetomotive force must alsol reverse when the fielddetector unit is reversed in its alignment with the field.

While balancing or polarizing means may be employed to achieve theseobjectives it is preferable that they be achieved by means of some typeof phase modulator as, for example, the phase modulators 32 and 33`shown in FIG. 2. Phase modulators of this general type are well known inthe art and require no detailed description. Briefly, however, they maybe described as comprising a two-coil transformer 34 with a center tapsecondary connected in balanced relation with two diodes 38i which maybe included in a single envelope. The common leg of the balanced networkincludes a transformer 36 and the output circuit comprises acenter-tapped resistor or potentiometer 40. If voltages of the samefrequency are applied to the primaries of transformers 34 and 36 theoutput direct current voltage across the center-tapped resistor 4.1iwill vary in magnitude and polarity in proportion to the magnitudes ofthese voltages and the phase angle existing between them. Thisdescription of the phase modulator 32 applies equally to the phasemodulator 33.

The circuits for energizing the gradiometer-magne-- torneters Z and 3have already been described as being substantially identical with thosepreviously described in connection with FIG. l. However, the detectioncircuit for the second harmonic voltages is slightly different from thatof FIG. '1. In FIG. 2 the resultant second harmonic voltage appearingacross conductors 20 and 21 is fed into a bridge network comprising theend elements 4Z, 43 of the band-pass filter 9, the reactors 45 and 46,the reactors `49 and 50 and the potentiometers 51 and 52. The endsections 42 and 43 of the band-pass filter 9 provide a low impedancepath to ground for the second harmonic currents and consequentlyterminals 10 and 11 may be regarded as being effectively grounded forvoltages of second harmonic frequency. Thus in effect the secondharmonic current generated in the coil 6 of gradiometermagnetometer 2 isconfined to a series circuit beginning with ground through this coil 6of magnetometer 2, line 20, reactor 49 and potentiometer 51, reactor 45and phase adjusting resistor 48, conductor 14 to ground through the endfilter element `42.

Similarly, the second harmonic current generated in the exciting windingof magnetometer 3 is confined to a series `circuit starting from groundthrough this pick-up winding 6 of magnetometer 3, conductor 21, reactor5f) and potentiometer 52, reactor 46 and back to ground through the endfilter element 43. It is obvious therefore that if the field detectorunit is in a uniform field and the magnetometers 2 and 3 areelectrically identical so that their generated second harmonic voltagesare equal the potentiometers 51 and 52 may be adjusted so that thevoltage appearing across the primary windings 53, 54 of transformer 23will be zero. It also will be evident that should there be a slightinequality in these voltages when in a uniform field that thisinequality may be adjusted by adjusting either potentiometer 5-1 or 52.

In practice these potentiometers 51 and 52 are adjusted to cause thegradiometer-magnetometers 2 and 3 to effectively have equal outputs forequal field increments. Then the slight inequalities in residualmagnetism existing between gradiometer-magnetorneters 2 and 3 may beadjusted by means of potentiometer 56 which is supplied from a directcurrent source 58 through a high resistance 57. The adjustment of thispotentiometer adjusts the relative amounts of direct current which mayflow from battery 58 through the exciting windings 6, 6 of thegradiometer-magnetometers 2 and 3i. These circuits may be traced fromrground through battery 58, high resistance 57, potentiometer 56 wherethe current divides, part of it going through winding 53, throughpotentiometer 51, conductor to the coil 6 of gradiom- .eter-magnetometer2 and back to the battery 58 through ground. The divided current frompotentiometer 56 passes through Winding 54 and follows a similar paththrough potentiometer `52, conductor Z1, the exciting winding 6 ofgradiometer-magnetometer 3 and back to the source 58 through ground.This direct current flowing through the exciting windings of themagnetometers 2 and 3 produces a magnetomotive force which may be usedto further balance out inaccuracies in construction and may be used tobring the indication of the gradient indicator or recorder 27 to Zerowhen the field detector unit 1 is in a uniform field.

When all of the adjustments for balance are made so that when the fielddetector unit 1 is in a uniform field, the voltage appearing across theprimary 'windings 53 and 54 is zero. Then should the field becomenon-uniform so as to present a magnetic gradient between the twomagnetometers 2 and 3, this voltage across the primaries 53 and 54 willobviously become unequal and the phase of this voltage will depend uponthe direction of the gradient effecting the twlo gradient magnetometers2 and 3. The two primary windings 53 and 54 are connected togetherthrough a capacitor.

The secondary winding 55 of `transformer 23 is connected to a secondharmonic filter 24 the output of which is amplified by a tuned amplifier59 and applied to the primary winding of ytransformer 314 in the phasemodulator 32. In order to properly operate the phase modulator 32 it isnecessary to have a reference voltage of the same frequency and of fixedphase applied to the primary of transformer 36. it is practicallynecessary that this voltage be derived from or closely synchronized withthe same source that is used to energize the gradient magnetometers 2and 3. In FIG. 2 this is done by connecting a frequency doubler `64 toterminal 12 of the band-pass filter 9. The output of this frequencydoubler actually contains a large number of harmonics but the secondharmonic is selected by a tuned circuit 65 and amplified by twoamplifiers 66 and 67. The output of the amplifier 66 is substantiallyconstant in magnitude and `is applied to the primary of transformer 36thereby pro- 8 viding the second harmonic reference voltage formodulator 32.

With the connections just `described it will be evident that the `directcurrent output voltage across the centertapped potentiometer 4f? will beproportional in magnitude to the magnitude of the second harmonicresultant voltage coming from the gradiometer-magnetometers 2 and 3since it is a voltage proportional to this resultant voltage which isapplied to the primary of transformer 34. Also it will be evident thatshould this voltage reverse in phase the polarity of the direct currentvoltage across potentiometer 40 also reverses in polarity. This directcurrent voltage from potentiometer ifi is amplified by a direct currentamplifier 6() and indicated by a gradient indicator or recorder 27. This:gradient indicator or recorder should be a center indicating orrecording instument so that with changes in polarity the indicationswill be on opposite sides of a mid-position. Consequent* ly, thisindicator or recorder 27 will indicate not only the magnitude of thefield gradient but also its direction.

The average field gradiometer 4 develops a second harmonic voltageproportional to the magnitude of the average field and its polarity orphase will be controlled by the direction of the magnetic fieldcomponent to be measured. This second harmonic voltage is transmittedover conductor 212 through inductor d'7, conductor 16, and to groundthrough the end filter element 44 which is similar to elements 42 and43. This second harmonic voltage therefore appears as a drop acrossinductor 47 and is applied through conductor 19 to the input circuits ofa two-kilocycle filter 28. This filter is a band-pass filter permittingthe passage only of the second harmonic of two kilocycles and the outputof this filter is amplified by an alternating current amplifier 61 andapplied to the primary `winding of a transformer 35 which corresponds infunction to the transformer 34 in the phase modulator 32. The phasemodulator 33 acts in exactly the same manner as the phase modulator 32and the reference voltage is derived from the same source through theamplifier 67 to the primary winding of transformer 37. The rectifiedoutput appears across the potentiometer 1211 and is amplified by adirect icurrent amplifier 62, `the output of which is passed through theaverage field indicator 3i and the low-pass lter 63 to the feedbackcoils 7, 7, 7 in the field detector unit 1. `By reason of the use of thephase modulator 33 this feedback current which is obtained from thedirect current amplifier 62 reverses in polarity ywhenever the directionof the magnetic field to be measured reverses in sign. As previouslystated this is the necessary requirement in order [that the feedbackmagnetomotive force `generated in the feedback coils 7, 7, 7 will havethe proper direction to oppose the applied field. Since this currentwill reverse in sign the average field indicator 31 should be a centerzero type instrument so that it will indicate the direction as well alsthe magnitude of the field to be measured.

IEIGS. 3 and 4 taken together shew in greater detail the variouscircuits shown in block form in FiG. 2. The oscillator 8 is shown inFIG. 3 and comprises tubes 70 and 71 connected in balanced relation, thecircuits themselves being substantially identical with those disclosedin British Patent 149,018 to Eccles-Jordan, complete accepted August 12,1920. The output of this oscillator may be coupled to the band-passfilter 9 through a bal anced power amplifier 72, 73 and transformer 74.These circuits are entirely conventional and require no detaileddescription. The band-pass filter 9 is of the three-channel type havingits output end sections of the form shown in FIG. 2. The connectionsfrom this band-pass filter to the field detector unit 1 are shownsubstantially as they appear in FIG. 2 and the corresponding parts inthese two figures bear the same reference numerals. The second harmonicoutput from the secondary 55 of transformer 23 is` connected to theinput circuit of band-pass filter 24 in FIG. 4 through section 75A ofthe multiposition field switch 75. This circuit may be traced from theupper end of secondary 55 down through the upper deck 75A of themultiposition switch 75, through the brush of this switch to the inputterminals of filter 24 in fFIG. 4. The output of this band-pass filter24 is connected to a tuned amplifier 59 which is shown in FIG. 4 ashaving two stages of amplification interconnected in a conventionalmanner with a tuned network. The output of this amplifier is connectedto the primary of transformer 34 in the phase modulator 32 just as shownin FIG. 2. The output of the phase modulator is taken from across thepotentiometer 40 and is applied to the input circuit 80 of the directcurrent amplifier 60.. The output of this direct current amplifier istaken from across its cathode circuit resistors by Way of conductors 68to switch deck 75D and 75E of the multiposition switch 75 in FIG. 3.From the brushes of these two decks the circuit is carried to thegradient indicator or recorder 27. An alternating current output is alsotaken from the plate circuit of this amplifier for test purposes. Thisoutput is taken from transformer 76 to a telephone receiver 69, thepurpose for which will be explained in more detail later.

The frequency doubler 64 of FIG. 4 has its input circuit coupled throughconductor 86 and inductor 47 to the output terminal 12 of the band-passfilter 9 in FIG. 3. This frequency doubler is of the type wherein thebias resistance is adjusted to the point where a maximum amount ofsecond harmonic energy is obtained from the plate circuit. This secondharmonic energy is coupled to the input circuits of amplifiers 66 and 67through the tuned interstage network 65. Amplifiers 66 and 67 are simpleamplifiers Whose output circuits are connected to the primaries oftransformers 36 and 37, respectively. These transformers are connectedin the common leg of the phase modulators 32 and 33 as was described inconnection with FdG. 2.

The direct current output from phase modulator 33 is coupled to theinput circuit of a direct current amplifier 62 through a low-pass filternetwork 78. In FIG. 2 this low-pass filter network 78 is regarded aspart of the input circuits of direct current amplifier 62. The outputfrom the direct current amplifier 62 is taken from across the cathodecircuit resistors by way of conductors 77 in FIG. 4 to the low-passfilter network 63 in viFIG. 3. This circuit also includes in seriestherewith the average field indicator 31. These circuits aresubstantially identical with those previously described in connectionwith FIG. 2.

The apparatus as thus far described in connection with FIGS. 2, 3 and 4may be adjusted and operated as follows: With the power all turned onand the tubes permitted sufficient time to warm up and become stabilizedthe field switch 75 shown in FIG. 3 should be set to the :+B position.In this position it will be noted that one of the leads from thegradient indicator or recorder 27 is connected to ground through theswitch deck 75E of the field switch and a resistor, while the otherterminal is connected through switch deck 75D, resistors 82 and 83 and achoke 84 to the positive terminal of the power supply unit which isschematically illustrated in FIG. 3 as a battery B. ln this position thegradient indicator or recorder will indicate the voltage of the powersupply source which, for proper operation should be adjusted, ifnecessary, to some predetermined voltage value. It will also be notedthat at this position the switch deck 75B places ground on the controlgrid of the second stage of the alternating current amplifier 61 therebyreducing to Zero the feedback current through the feedback coils 7, 7, 7of the field detector unit 1. The brush of the switch deck 75A alsoplaces ground on the input terminals of the band-pass filter 24 so thatthere will be no input to the tuned amplifier 59 and phase modulator 32.Since there is no transmission through the alternating current amplifier61 by reason of having the control grid of the second stage groundedthere is no alternating cur- 1Q rent input to the primary of transformer35 of phase modulator 33 and consequently the average field indicator 31should read 0. Proper balance is accomplished by adjusting the slider ofpotentiometer 41 until meter 31 reads 0.

The field control switch should then be set at position GR which is theposition normally used for making the gradiometer measurement. The scaleswitch found in the grid circuit of the second stage of the tunedamplifier 59 as shown in FIG. 4 should then be placed to the point ofmaximum sensitivity, that is, to its upper contact as shown in FIG. 4.With these adjustments the gradiometer-indicator 27 should be adjustedto read zero by adjusting the potentiometer 56. As previously statedthis potentiometer adjusts the relative amount of direct current whichmay be applied to the two gradiometerdetector magnetometers 2 and 3. Thescale switch should then be adjusted to minimum sensitivity, that is, toits lowermost Contact and potentiometer 4f) located in the phasemodulator 32 should be adjusted to give a zero reading on thegradiometer indicator 27. Potentiometer in the direct current amplifier60 should next be adjusted to give minimum tone in the telephonereceiver 69. This potentiometer S0 is fed by some 100G-cycle energy fromterminal 12 of the band-pass filter 9 in FIG. 3 by way of conductors 85,86 and 87. This tends to balance out the alternating current componentin the direct current amplifier 60. The scale switch in amplifier 59 maythen be returned to maximum sensitivity, that is, its upper contact.Readjustment of potentiometer 56 may now be necessary to bring the twogradiometermagnetometers 2 and 3 to more perfect balance.

The apparatus may be calibrated for a predetermined field gradient byapplying a known field gradient to the field detector unit 1 andobserving the response of the gradient-indicator 27. Special circuits,however, are provided in the apparatus disclosed for `artificiallyproducing the known field gradient. This comprises unbalancing theamount of direct current applied to the exciting windings 6, 6 of thetwo gradiometer-magnetometers 2 and 3. The standardization orcalibration is effected by first adjusting the field switch to theposition -l-B and noting the reading of the gradiometer-indicator, thenthe field switch is placed on its position CAL which is the calibrateposition. lIt will be noted that in this latter position the circuitsare identical with the circuit set up with the switch at the GR positionfor normal operation except that switch deck 75C connects ian additionalresistor 88 into a network comprising resistors 89, 90, 57 andpotentiometer 56. The magnitudes of these resistors `are all selected tobe of such size as to` place a predetermined amount of unbalance directcurrent in the gradiometerrnagnetometers 2 and 3 thus artificiallyproducing a known field gradient as, for example, about 22 gamma perfoot. In order that the gradiometer-indicator 27 will be readingproperly for this known field, the rheostat 79 in the cathode circuit ofthe second stage of amplifier 59 shown in FIG. 4 should be adjusteduntil the gradiometerindicator 27 reads the same as the reading notedwhen the field switch was on position +B. It is obvious that other meansmay be employed to standardize or calibrate the gradiometer-indicator.However, the one herein disclosed is self-contained and easily used.

The field switch should be returned to its GR position which is itsposition for normal operation. The gradiometer-detector should then beoriented into a substantially uniform field having a large component ofnot less than 0.2 gauss in the direction of the principal axis of thedetector unit 1. 'Ihis may be observed by observing the reading of theaverage field indicator which may be calibrated in terms of gauss. Whilein this position the field switch should be adjusted to the position Fland rheostat 81, located in the cathode circuit of the first stage ofarnplifier 61, adjusted until a minimum reading is observed on thegradiometer-indicator 27. This adjusts the feedback current through thefeedback coils 7 in the field de- 1 "l tector unit 1 so that the net eldin gradiometer-element 3 is reduced to zero.

The tield switch should then be placed on the GR position and with thefield detector unit still oriented into a large axial field of at last0.2 gauss, potentiometer 51 shown in FIG. 3 should be adjusted to give aminimum change in reading on the gradient-indicator 27 when the fieldswitch is switched from position GR to position CHK. It will be notedthat with the field switch on position CHK the circuits are identical tothose set up when the eld switch is on position GR with the singleexception that the switch deck 75B connects the grid of the second stageof ampliiier 61 to terminal 90 instead of terminal 89 of the resistancenetwork 91, 92 shown in amplier 61 of FIG. 4. This obviously reduces thcgain of the feedback amplifier so as to add a predetermined amount ofthe ambient magnetic field to each magnetometer. The adjustment ofpotentiometer 51 adjusts the gradiometer-magnetorneters 2 and 3 so thatthey have equal sensitivities for equal eld increments.

The next step in the adjustment procedure is to adjust thegradiometer-magnetometer elements 2 and 3 so that their principal axesare in exact alignment or at least parallel and in substantialcollinearity. This is accomplished by replacing the iield switch 75 onposition G'R and with the scale switch in amplifier 59 at its point ofmaximum sensitivity, the iield detector unit 1 should be rotated intozero eld, that is, 90 degrees with respect to the direction of theambient eld. rThis position may be determined by observing the readingof the average field indicator 31 which should read 0 in this position.It should be understood that in this position the average iieldindicator 31 will read 0 when the average field magnetometer 4 has itsprincipal axis perpendicular with the ambient tield and the problem isto align the gradiometer-magnetometers 2 and 3 so their principal axesare likewise perpendicular to the ambient lield. This is accomplished bymechanically adjusting the alignment of the gradiorneter-magnetometerelements Z and 3 until the entire eld detector unit 1 may be rotatedabout its own axis without any variation in the reading of thegradient-indicator 27. The mechanical means for making this adjustmentwill be described in greater particularity later in connection with'FlGi 5 to 8, inclusive.

Any inequality in the feedback fields produced by the feedback coils 7,surrounding the gradiometer-detector elements 2 and 3 may be observed byorienting the lield detector unit 1 into alignment with the direction ofthe ambient held so that the average iield indicator reads at leastabout 0.2 gauss. With the eld switch 75 on its gradiometer position Giland with the scale switch at its point of maximum sensitivity the elddetector unit 1 should be reversed end for end. if there is any changein the gradiometer reading `from one position to the other this will becompensated for by changing the ad justment of the adjustment coil 7.This adjustment is made by simply either turning the `adjusting coil 7over or by changing its number of turns whichever is necessary.Sometimes both of these adjustments are necessary.

With the apparatus adjusted as just described and with the eld switch 75on the position GR the apparatus is ready for the measurement of lieldgradients. The average field indicator 31 will indicate the strength ofthe average field along the direction of the principal axis of the fielddetector unit 1, while the gradient-indicator or recorder 27 willindicate the gradient existing along the same axis between thegradient-magnetometers 2 and 3.

The magnetometer elements 2, 3 and 4 may be mounted in any type ofsupporting structure which preferably has some means of adjusting thealignment of the end elements. One such means disclosing a preferredform for supporting the elements in substantial collinearity is shown inFIGS. 5 to 8, inclusive. FIG. 5 shows an end View of the housing forsupporting these magnetorneter elements. 6A and 6B disclose a sectionview through FIG. 5. 'In FIGS. 6A and 6B the three magnetometer elements2, 3 and 4 are shown mounted within a rigid tube of non-magneticmaterial 93. These magnetometers are spaced a fixed distance apartdetermined by two spacers 94, 94. Supporting tube 93 is in turnsupported within yanother rigid tube 95 and spaced therefrom by means ofthree spacers 96, 96, 96. As the three magnetometers are substantiallyidentical in their construction the cross-section view of the mag-`uctomel'er Z shown in FiG. 6A is illustrative of all three.

lt will be noted that the section view of magnetometer 2 in FIG. 6Ashows the core of magnetic material 5 in dotted outline and surroundedby the exciting winding 6 which, although it is shown in two sections,may be wound only in one section or in more than two. The winding 6 isplaced upon a non-conductive, non-magnetic spool 98 within which themagnetic core 5 is securely retained. This spool is then placed within atube of `insulating material 99 to which is iirmly attached twospoolheads 100, 100. These spoolheads have an outside diametersuiiicient to cause them to t snugly within the supporting tube 93 andcontain around their periphery a series of slots 101 which lendsflexibility to the spoolheads to insure secure Contact with the innersurface of the supporting tube 93. The feedback coil 7 is wound aroundthe insulating tube 99. The electrical connections for thesemagnetometers are not shown in these figures but the connection sarebrought through the spoolheads and out through a cable connection 102shown at the right end of FIG. 6B. These connections are made inaccordance with FIGS. l, 2, 3 and 4 already described.

In order to prevent the entrance of moisture into the gradiometerassembly end caps 103 and 104 are provided for the ends of the outersupporting tube 95. A dessicant may also be used to insure drynessinside the enclosure. These end caps contain plugs 105 and 106respectively wihch may be removed in order to permit the entry of ascrew-driver for adjusting the alignment of the twogradiometer-magnetometers 2 and 3. The holes containing these plugs 105and 106 are placed opposite two adjustment screws 107 and 108 which areengaged by the screw-driver in making the alignment adjustment. As thismechanical adjustment requires flexibility in the vicinity of the endelements it is necessary that the inner supporting tube 93 be madesomewhat flexible at its ends. This is accomplished by a series oflongitudinal slots 97 which are cut in the ends of the supporting tube93 of length substantially coextensive with the length of themagnetometer elements. The adjustments are effected by adjusting screws109 and 110. Screws 109, of which there are two diametrically opposed ineach of the ends, are threaded directly in a non-magnetic lock 111. lnFIG. 6B it is obvious that by the adjustment of screws 109 the right endof magnetometer 3 may be moved vertically so as to change the axialalignment thereof with respect to the average field magnetometer Y4. InFlG. 6A the adjustment screws 109 are horizontal and consequently theadjustment of these screws will adjust the horizontal alignment of themagnetometer 2 with respect to the average lield magnetometer 4.

The adjustment screws 110 are threaded directly in the block 111 butrather are they threaded int-o a worm gear 112 best shown in FIGS. 7 and8. This worm gear is rotated by means of a worm 113, also shown in FiG.7. The end of the shaft for worm 113 is slotted to receive ascrew-driver as shown at 107 in FIG. 7 and also by the same referencenumeral in FIG. 6A. It is to be understood that the mechanicalconstruction of this adjustment is identical for each end, that is tosay, the adjustment screw 108 operates upon adjustment screw 110 in FIG.6B in just the same manner as adjustment screw 107 opcrates theadjustment screw 110 in FIG. 6A.

In FIG. 7 the adjustment screws 109, 109' are threaded into block 111 asshown in the cut-away View on the left side. When these screws areproperly adjusted they may be keyed in place by means of pins 115 or byany other convenient means of locking the screws 109 against turning. InFIG. 8 the preferred construction for the adjustment screw 110 is moreclearly observed. It will be noted that screw 110 is threaded internallyto receive a similar external thread 114 on the rod 110. The threadedportion 114 is also long enough to include the worm gear 112 which isalso threaded internally to take this same thread. Longitudinal slots116 are cut in rod 110A throughout a substantial portion of the threadedpart 114 where it passes through the worm gear 112. Rod 110 also has ahole drilled centrally from its lower end. The exact construction ofthis hole need not be described in detail as this type of constructionis well known and has been used for a number of years in expansionreamers. The purpose of the slot 116 and this hole is to permit athreaded plunger 118 to be turned down into this hole and to slightlyexpand the rod 110 under the worm gear 112 thereby eliminating anybacklash. The upper portion of rod 110 is flattened as indicated clearlyin FIGS. 7 and 8. These ilats cooperate with a slot 119 cut in the endof the inner supporting tube 93 and serves to prevent the rotation ofthe shaft 110 when adjustments are being made by turning the worm gear112. An adjustment sleeve 120 is threaded into block 111 and whenproperly adjusted prevents vertical motion of the worm gear 112 andconsequently any vertical motion of the rod .110 and adjustment screw110.

With the parts assembled as shown in FIG. 8 the adjustment sleeve 120 isbrought down against the worm gear 11.2 to take up any vertical motionof the gear. Adjustxnent screw 110 is rotated so that its lower endbears against the inner surface of the outer supporting tube 95, whilethe upper end of rod 110 is similarly brought to bear against the innersurface of tube 95. Adjustment screw 110 may then be keyed againstrotation with respect to the rod 110 by means of a pin 121. While thispin prevents the relative rotation between screw 110 and shaft 110 itdoes not prevent the vetrical adjustment of this assembly with respectto block 111 by reason of a slot 122 milled into the ends of block 111.This slot 122 may be more easily observed in FIG. 7.

While it has been stated that it is desirable to accurately align theprincipal magnetic axes of gradiometer-magnetometers 2 and 3 inalignment with the principal magnetic axis of the average fieldmagnetometer 4, it is very much more important to the gradiornetermeasurement that the principal axes of the gradiometer-magnetometers 2and 3 be in exact alignment with each other, either in exactcollinear-ity or in substantial collinearity with their principal axesparallel. Consequently, the alignment mechanisms for the two ends aremounted 90 degrees with respect to each other. The adjusting screws 109at both ends are then adjusted as accurately as possible and keyed inplace by the pins 115. Thereafter the adjustments may be made throughthe plugs 105, 106 by means of a screwdriver cooperating with theadjustment screws 107 and 108. These screws it will be rememberedcomprise a shaft with a worm which cooperates with the worm gear 112.Should a slight vertical misalignment exist in the magnetometer 3 ofFIG. 6B the adjustment of the adjusting screw 107 of FIG. 6A can bringthe principal axis of the gradiometer-magnetometcr 2 into horizontalparallelism with the principal axis of gradiometer-magnetometer 3.Similarly, the adjustment of screw 1.03 in FIG. 6B will move theprincipal axis of the niagnetometer 3 horizontally until it is inalignment with or until it lies in a Vertical plane which is parallel tothe vertical plane in which the principal axis of thegradiometer-magnetometer 2 lies.

What is claimed is:

l. In a magnetic gradiometer system for measuring the gradient of acomponent of a magnetic iield, the dield detector unit comprising threemagnetometer elements each comprising a core of magnetic material withwindings thereon and each having a principal magnetic axis, a spoolheadon each end of each magnetometer, a supporting means comprising astraight tube of nonmagnetic material closely surrounding and iittingsaid spoolhead, spacers of non-magnetic material inserted be tweenadjacent magnetometers within the tube to `fix the spacing between them,a exible portion at each end of said tube adjacent two of themagnetometers, and a mechanical adjusting means for bending said lexibleportion to adjust the alignment of the principal axes of saidmagnetometers relative to each other.

2. In a magnetic gradiometer system for measuring the gradient of acomponent of a magnetic field, the tield detector unit comprising threemagnetometer elements each comprising a core of magnetic material withwindings thereon and each having a principal magnetic axis, a spoolheadon each end of each magnetometer, a supporting means comprising astraight tube of non-magnetic material closely surrounding and fittingsaid spoolhead, spacers of non-magnetic material inserted betweenadjacent magnetometers within the tube to ix the spacing between them, aflexible portion at each end of said tube adjacent two of themagnetometers, a non-magnetic rigid outer tube coaxial with andsurrounding said lirst tube, non-magnetic tubular spacers closelyfitting between said two tubes and spaced from the flexible portions ofthe first tube, and a mechanical adjusting means mounted to the end ofthe first tube with a plurality of adjusting legs projecting therefromto engage the outer tube whereby the liexible ends of the first tube maybe adjustably flexed to adjust the alignment of the principal axes ofsaid magnetometers relative to each other.

3. The combination in accordance with claim 2 and end caps enclosing theends of said outer tube, and removable plugs in the caps to provideaccess to the mechanical adjusting means.

References Cited in the file of this patent UNITED STATES PATENTS2,379,716 Hull July 3, 1945 2,407,202 Vacquier Sept. 3, 1946 FOREIGNPATENTS 521,339 Great Britain May 20, 1940

